there are lots of things you may want to know e.g.
- metering patterns (spot, centre and matrix or evaluative)
- does it have manual mode
- shutter speeds (slowest to fastest)
- lens compatibility i.e. can older lenses be used with the body and will newer lenses still be usable.
- one thing I always look for is a Depth-of-field (DOF) preview feature. I find this very useful, specially for close up and portrait shots.
- what accessories can the body use
- will the camera work without batteries (mechanical or electronic shutter, metering operation doesn't count)
- focusing screen, is it ground glass or is there a split image screen (depends which one you're more comfortable with)
- camera weight (with batteries)
- what kind of batteries does it use, are they easily available
- film DX override
a lot man...i just add on to what patch mentioned, as i recall on the spot:
- flash: mount a compatible hotshoe flash and mount a zoom lens also, then check all the functions are working
- AF; must not be sluggish
- shutter: put in new batteries and set to various speeds: 1/2000, 1/500, 1/125, 1/60, 1/30, 1/15, 1/8, 1/4, 1/2, 1", 2", 8"....put your ear close to the cam body, fire, and listen: all of the mentioned speeds must sound like themselves. ie eg 1/2000 shouldnt sound sluggish like say 1/8.
-CF: custom functions must work as mentioned in the manual.